Centauri Dreams
Imagining and Planning Interstellar Exploration
Where Are the Jupiter Analogs?
Are Solar Systems like ours commonplace? One way of answering this is to look at the role of planets like Jupiter, which may have helped to determine the habitability of the inner planets. But worlds like Jupiter in orbits around 5 AU do not appear to be the norm, as Andrew LePage points out in this discussion of a new exoplanet find. LePage, publisher of an essential site on exoplanet detection (www.DrewExMachina.com) is also a Senior Project Scientist at Visidyne, Inc. in Boston. Today he shows us what we know and just how much we still need to clarify about the occurence of planets like Jupiter and their role in system habitability.
By Andrew J. LePage
A couple of decades ago, astronomers thought they had planetary systems figured out: they consisted of a more or less orderly set of worlds orbiting in the same plane with small rocky worlds close in and much larger, volatile-rich planets orbiting farther out beyond the “snow line” where plentiful water freezes into solid ice. Along with this model came the view among some that the presence of large Jupiter-like planets was not only likely but required to deliver water and clear out potential impact hazards to ensure the habitability of smaller rocky worlds orbiting inside a star’s habitable zone. Coupled with the Copernican principle that implies that there is nothing special about our Solar System, it was expected that extrasolar planetary systems would have similar architectures and possess “Jupiter analogs”.
But with the discovery of the first extrasolar planet orbiting a main sequence star back in 1995, this orderly view of planetary systems was called into question. This first exoplanet, 51 Pegasi b, was a Jupiter-mass world in a four-day orbit only 8 million kilometers from its sun. This “hot Jupiter” and a host of other extrasolar giant planets (EGPs) discovered since with a wide range of orbital radii clearly demonstrated that other arrangements of planetary systems are possible and that Jupiter analogs might not be the norm after all. Unfortunately, getting a clear picture of exoplanetary systems has been difficult because of the detection biases of the most often used detection techniques (i.e. precision radial velocity measurements and transit observations) clearly favor finding large planets in small orbits with short periods. But with two (and sometimes more) decades of data from various long-running radial velocity surveys now available for analysis, this is beginning to change as it now becomes possible to detect EGPs with orbital periods of a decade or more.
Earlier this month, the team responsible for Lick-Carnegie Exoplanet Survey announced the latest discovery of a Jupiter analog in a paper accepted for publication in The Astrophysical Journal. What makes this discovery all the more interesting is that the lead author, Dominick Rowan, is a senior at Byram Hills High School in Armonk, New York. Rowan recently won individual top honors in the Regional Finals of the Siemens Competition in Math, Science & Technology as a result of his work described in this paper. The newest Jupiter analog found by Rowan et al. orbits the Sun-like star HD 32963 about 120 light years away. With a mass estimated to be 94% that of the Sun, this star has a luminosity of about 90% of the Sun’s and an estimated age of around five billion years.
To find the new Jupiter analog, designated HD 32963b, a total of 199 radial velocity measurements acquired over 16 years using HIRES (High Resolution Echelle Spectrometer) on the 10-meter Keck I telescope at Mauna Kea, Hawaii were analyzed. These data were placed into two-hour bins to create 109 individual radial velocity measurements with a typical uncertainty of ±1.2 meters/second. A clear signal with a semiamplitude of 11 meters/second and a period of 6.49 years was seen in the data with only a 2×10-5 false alarm probability. This signal indicates the presence of a planet in a nearly circular 3.4 AU orbit – only a touch smaller than Jupiter’s 5.2 AU orbit around the Sun. Since the tilt of the new planet’s orbit with respect to the plane of the sky is not known, only the minimum mass or Mpsini of 0.70 times that of Jupiter (or MJ) can be determined using radial velocity measurements alone. By assuming a randomly oriented orbit, there is less than a one in three chance that HD 32963b is more massive than Jupiter.
Rowan et al. took their analysis one step further and examined the data from the Lick-Carnegie Exoplanet Survey to determine the occurrence rate of Jupiter analogs around Sun-like stars. For the purpose of this analysis, a Jupiter analog was defined as an EGP with a mass in the 0.3 to 3 MJ range orbiting a Sun-like star with an eccentricity less than 0.3 and a period of between 5 to 15 years (which corresponds roughly to orbital radii in the 3 to 6 AU range). The new find by Rowan et al. qualifies as a Jupiter analog by this definition.
Image: Jupiter dominates our Solar System and may have had a role to play in the habitability of our own planet. We’re only now learning, however, how common such worlds are in orbits comparable to our own Jupiter’s at 5 AU. Credit: NASA/JPL/University of Arizona.
A review of the Exoplanet Data Explorer in August 2015 revealed 21 EGPs that met the working definition for Jupiter analog. Of these, eight published exoplanets that met the baseline requirements were found among the 1,120 Sun-like stars in the Lick-Carnegie Exoplanet Survey, yielding a raw frequency rate of 0.71%. In order to turn this raw number into a meaningful occurrence rate, the detection efficiency of the radial velocity survey for this class of planet must be taken into account. To accomplish that, Rowan et al. created synthetic radial velocity data sets for each star in their survey representing 320,000 different combinations of various planetary mass and orbital parameters. The ability of their analysis algorithms to detect these velocity variations through the noise in the data was then gauged to determine the detection efficiency.
The analysis by Rowan et al. found that the occurrence rate of Jupiter analogs orbiting Sun-like stars as defined here was approximately 3%. Making reasonable assumptions about the possible distribution of EGP properties, the rate can not be less than about 1% nor greater than around 4%. These results roughly agree with earlier studies based on microlensing and long-term radial velocity surveys. This suggests, contrary to earlier expectations, that Jupiter analogs are not common. This finding implies that either planet migration mechanisms from the snow line are very efficient at moving EGPs into smaller orbits or that EGPs have a difficult time forming at distances of about 5 AU.
But before the alarms are sounded by “rare Earth” advocates about the relative rarity of Jupiter analogs, it remains to be seen just how vital Jupiter analogs are to planetary habitability given the spectrum of architectures observed in exoplanet surveys to date. Even if Jupiter analogs prove to be required, current surveys have yet to search effectively for smaller Neptune to Saturn-size worlds at distances of about 5 AU or for EGPs at greater distances which may suffice as “Jupiter surrogates”. Only continued collection of radial velocity data along with new surveys, such as the current Gaia astrometry mission and the new generation of telescopes to image exoplanets directly, will allow us to fill in the missing pieces of our knowledge of exoplanetary systems. Thankfully this ongoing search provides opportunities for aspiring young scientists like Dominick Rowan.
The paper is Rowan et al. “The Lick-Carnegie Exoplanet Survey: HD32963 — A New Jupiter Analog Orbiting a Sun-like Star” accepted for publication in The Astrophysical Journal (preprint).
E-22: The Last Hurrah at Enceladus
It’s the end of an era. On Saturday December 19, the Cassini Saturn orbiter will make its final close pass by Enceladus. This doesn’t mark the end of Cassini itself, which still has work to do especially with regard to Titan, but it does mean the end, at least for now, of our close-up study of a remarkable phenomenon: The plumes of Enceladus, which Cassini itself discovered. We’ve gained priceless data through its flybys, helping us make the case for an internal ocean.
Cassini will continue to observe Enceladus until mission’s end, but only from much greater distances. In fact, as this JPL news release explains, the closest Cassini will come to Enceladus after Saturday is about four times the distance of the upcoming flyby. Nor will the Saturday event be as close a pass as Cassini’s dive through the south polar plume on October 28. That one took the spacecraft within a scant 49 kilometers of the surface, returning data that is still undergoing analysis as we probe the plume’s activity. A key issue: Is there hydrogen gas here, which would offer yet more evidence for active hydrothermal systems on the seafloor?
Image: An animation showing the upcoming December 19 flyby of Enceladus, in which Cassini’s composite infrared spectrometer instrument will observe the moon’s south polar terrain. Credit: NASA/JPL.
The Saturday event will be all about measuring the moon’s heat, a critical factor in making sense of the factors driving the spray of particles and gas from the ocean beneath. 5000 kilometers turns out to be about the right distance to allow the best working of Cassini’s Composite Infrared Spectrometer (CIRS) as it maps the heat flow across the southern polar region, according to Mike Flasar, CIRS team lead at NASA GSFC:
“The distance of this flyby is in the sweet spot for us to map the heat coming from within Enceladus — not too close, and not too far away. It allows us to map a good portion of the intriguing south polar region at good resolution.”
Image: An exciting chapter of space exploration history will come to a close as NASA’s Cassini spacecraft makes its final close flyby of Saturn’s active, ocean-bearing moon Enceladus. The spacecraft is scheduled to fly past the icy moon at a distance of 4,999 kilometers on Dec. 19 at 0949 PST (1749 UTC). Although Cassini will continue to observe Enceladus for the remainder of its mission (through Sept. 2017), its next-closest encounter with the moon will be at a distance more than four times farther away. The focus of the Dec. 19 encounter will be on measuring how much heat is coming through the ice from the moon’s interior — an important consideration for understanding what is driving its surprising geyser activity, which Cassini discovered in 2005. Credit: NASA/JPL-CalTech.
Keep in mind that the south polar region of Enceladus was well lit when Cassini arrived at Saturn in 2004, but at present the area is in winter darkness, making these heat studies that much easier to complete absent the heat of the Sun. By mission’s end, we will have data on six years of winter darkness in the south polar area. The discovery of geologic activity caused the mission’s flight plan to be changed to make Enceladus a prime target of operations. Of all the gifts Cassini has given us, finding a global ocean beneath the ice glitters the most brightly.
Naming New Worlds
I can only wonder what Miguel de Cervantes Saavedra would have thought of the idea that a distant star would one day be named for him. I wonder, too, what the Spanish novelist (1547-1616) would have made of the idea that planets circled other stars, and that planets around the star named for him would have names taken from his most famous work, Don Quixote. Maybe the great character of the book’s title, obsessed with tales of chivalry, would have been unhinged enough to take things like other solar systems in stride.
We have the NameExoWorlds contest to thank for these speculations. The contest, organized by the International Astronomical Union (IAU) gave the public the opportunity to choose the names of selected stars and planets. The star named for Cervantes is mu Arae (HD 160691), a G-class star about fifty light years out in the constellation Ara (the Altar). Here we’ve found three gas giant planets comparable to Jupiter as well as a ‘super-Earth.’ And frankly, as a reader who agrees with Schopenhauer that Don Quixote is one of the world’s great novels, I am delighted with what the public has chosen, at least for this star.
Image: Portrait of Miguel de Cervantes y Saavedra (1547-1615), by the artist Juan de Jauregui y Aguilar (circa 1583-1641). Credit: Bridgeman Art Library.
For mu Arae b becomes Quijote, while mu Arae c is Dulcinea. The system is rounded out with Rocinante (mu Arae d) and Sancho (mu Arae e). Names as enchanting as these, with roots in classic literature, are likely to stick. When the voting finished at the end of October, the IAU had received 573,242 votes, which went toward naming 14 host stars and 31 exoplanets. Names were submitted from astronomy organizations in 45 countries, everything from amateur astronomy clubs to universities and planetariums, drawn from a wild variety of sources.
Take Thestias, the planet depicted in the image below. The grandfather of Pollux, Thestias orbits the star of the same name (Pollux, already named, needs no further designation). In Greek mythology, Pollux and Castor were twin brothers known as the Dioscuri who became transformed at death into the constellation Gemini. The winning name came from SkyNet, an astronomy project based at the International Centre for Radio Astronomy Research (ICRAR) in Perth, Australia. The group, which includes over 200,000 volunteers globally, arrived at the submission by an internal vote, drawing on the idea of volunteer Rich Matthews.
Image: Artist’s impression of Thestias around its star Pollux. Credit: NASA/ESA and G. Bacon (STScI).
If mu Arae taps the late European Renaissance for its inspiration, 47 Ursae Majoris (46 light years out) draws on Thai folklore. The star receives the name Chalawan, while 47 Ursae Majoris b becomes Taphao Thong and 47 Ursae Majoris c is Taphao Kaew, these being two sisters in the story of a mythological crocodile king. The star upsilon Andromedae (an F-class star 44 light years away) becomes Titawin, a point of contact in Morocco between Spaniards and Arabs after the 8th Century. The planets upsilon Andromedae b, c and d become Saffar, Samh and Majriti, names drawn from astronomers and mathematicians notable in 11th Century Spain.
I think some of these names will last, but I’m not at all sure why the IAU chose to put out a call for new names for epsilon Eridani, at 10.5 light years one of the closest stars and a familiar name to generations of science fiction readers. I’m OK with giving epsilon Eridani b the name AEgir, which is drawn from Norse mythology (he was husband to Ran, the goddess of the sea), but changing the star epsilon Eridani to Ran is just not going to work, the original name being too widely circulated. It’s as odd as if we named the three Centauri stars anything other than their designation, the point being that by wide use, the designation and the name are one.
18 Delphini gets tagged Musica (lovely!), while its planet 18 Delphini b is now the wisely chosen Arion, a Greek musician whose tunes attracted the dolphins who saved him at sea. It’s also heartening to see the 55 Cancri planets named for great figures in astronomy including Galileo and Brahe, while the star itself becomes Copernicus. But is Poltergeist going to survive as the name of one of the pulsar planets (PSR 1257+12 c)? How about Spe for 14 Andromedae b?
My cavils aside, I love the idea of pulling the public into the naming of planets because we live in a world undergoing an unprecedented expansion of consciousness skyward. The great voyages of discovery have nothing on what we are doing now, pushing away from Sol to find planets at ever smaller and more Earth-like scale as we begin, in the tiniest way, the process of mapping the planetary systems of a galaxy of 200 billion stars. Ultimately, naming exoplanets will by necessity become an ad hoc process, to be resorted to as needed, because the number of planets will dwarf our lexicons. But we’re still getting used to that idea, and the NameExoWorlds contest has been a delightful way to bring visibility to the question.
‘Hot Jupiters’: Water Depletion Explained
Planets that transit across their star as seen from Earth allow us to use transmission spectroscopy to study their atmospheres. The idea is straightforward: Even though we can’t see the planet at optical wavelengths, we can examine the starlight that travels through its outer atmosphere during the transit. Each atmosphere leaves its own signature, and the atmospheres of some of the ‘hot Jupiters’ thus far studied have raised questions. Why do some of these worlds have less water than our models of their atmospheres would predict? Is this an indication that such planets formed in protoplanetary disks that were depleted of water?
A new study brings us some answers by going to work on eight hot Jupiters (WASP-6b, WASP-12b, WASP-17b, WASP-19b, WASP-31b, WASP-39b, HAT-P-1b and HAT-P-12b) using the Hubble Space Telescope. The worlds chosen here offer a wide range of temperature, surface gravity, mass and radii. All eight were observed at optical wavelengths using Hubble’s Space Telescope Imaging Spectrograph (STIS) instrument, while two of them (WASP-31b and HAT-P-1b) were also observed in the near infrared with Hubble’s Wide Field Camera 3.
But lead author David Sing (University of Exeter) and team did not stop there. The Hubble survey was bolstered by infrared data from the Spitzer Space Telescope, and the work folded in data from both HST and Spitzer on two of the most widely studied hot Jupiters, HD 209458b and HD 189733b. Although we’re only dealing with ten worlds, this turns out to be the largest spectroscopic catalog of exoplanet atmospheres yet assembled.
What we learn is that these planets show a continuum from completely clear to cloudy atmospheres. It turns out that the difference between a planet’s radius as measured at optical and infrared wavelengths allows us to distinguish between the two types. A cloudy planet shows up larger in visible light than it does in the infrared — at the latter wavelengths, we are looking deeper into the atmosphere. The paper notes the significance of this finding:
We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.
In other words, the clouds are the culprit, and ‘dry’ hot Jupiters are not depleted in water.
Image: This image shows an artist’s impression of the 10 hot Jupiter exoplanets studied by astronomer David Sing and his colleagues using the Hubble and Spitzer space telescopes. From top left to lower left, these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. Credit: NASA, ESA, D. Sing (University of Exeter).
A couple of things to note in the image. The colors are vivid but as this Hubblesite news release explains, they are purely for illustrative purposes. We have little data on the color of any of these worlds with the exception of HD 189733b, sometimes called the ‘blue planet.’ The cloud patterns shown here are also theoretical, based largely on what we see on Jupiter.
An interesting difference between hot Jupiters and brown dwarfs emerges in this paper. With the latter, which can have temperatures similar to hot Jupiters, we can go from warmer, cloudy L-class dwarfs to cooler T-dwarfs whose atmospheres are inferred to be clear — the paper calls this sequence ‘well defined.’ In contrast, hot Jupiters do not show a strong relationship between temperature and cloud formation, at least based on this sample, where both cloudy and not cloudy planets appear throughout the temperature range studied. The authors speculate on the difference:
We suggest that the difference between hot Jupiters and brown dwarfs is due to the vertical temperature structure of hot-Jupiter atmospheres. Hot Jupiters have very much steeper pressure-temperature profiles compared to isolated brown dwarfs, owing to the strong incident stellar flux heating the top of the planetary atmosphere… Since cloud condensation curves run nearly parallel to hot-Jupiter profiles, a relatively small temperature shift (about 100K) could easily move a cloud base by a factor of tens or hundreds in pressure, in or out of the visible atmosphere.
Moreover, some hot Jupiters are likely to have cloud materials that are cold-trapped deep within the atmosphere, and thus out of the range we can detect. The paper adds that hot Jupiters show a wider range of gravities and metallicities than brown dwarfs, factors that play a role in atmospheric circulation and condensation as well as temperature. As we learn more about how to distinguish between clear and cloudy atmospheres, we’ll strengthen our ability to focus on worlds with clear skies whose chemical abundances can be studied in greater detail.
The paper is Sing et al., “A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion,” published online in Nature 14 December 2015 (abstract).
SETI: Project Argus and the Long Stare
I think you’ll find Jon Lomberg’s new essay in Slate as interesting as I do. We Need a World Cup for SETI uses a familiar figure at many sports events — the guy in the stands holding up a Biblical reference on a poster — to dig into a far more interesting issue. How does one go about maximizing visibility? The guy with the sign knows how to do it and if we think about his methods, we can better understand SETI.
For as we think about radio and optical SETI, we’re usually looking for signals that have been intentionally sent. Here we run into the particularly tricky business of trying to understand the thinking of an alien being, but there are certain principles that may apply to any civilization trying to send out a beacon-like message. The message needs to be short, cheap, easy to find, and in a place where it’s likely to be seen. So what kind of beacon is this going to be?
We’ve discussed ‘Benford beacons’ in these pages before (see, among others in the archive, Detecting a ‘Funeral Pyre’ Beacon). A beacon announcing little more than ‘We Are Here’ could be used to attract the attention of any receiving culture, after which we (the receivers) would apply our resources to looking harder at the source. But from the standpoint of efficiency and economy, a brief, bright beacon is best. James and Gregory Benford have addressed the matter in two key papers cited at the end of this essay. Let me quote a brief bit of one:
No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs… But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon’s message arrived. This more sensible approach—a sort of Energy Star specification for SETI—would save enough power to keep the beacon running for millions of years.
The Benfords bring useful synergies to bear on the matter. Jim is a plasma physicist — he knows all about beaming — while Greg is both physicist and science fiction author, a man who has speculated for decades on the workings of extraterrestrial civilizations. Jim’s son Dominic, also active in the beacon work, is a physicist at NASA GSFC. Beacons like the Benfords are suggesting are different from the kind of directed beacon SETI has long looked for, one that demands intense focus on stars in the hopes of finding continually broadcasting signals. A Benford beacon puts out a signal we would perceive as intermittent, a brief pulse.
Image: What kind of signal to look for amidst 200 billion stars? Credit: Center for Planetary Science.
The Long Stare
You can see that this puts the premium on what SETI people call ‘dwell times,’ i.e., the time we look at a particular target. In his Slate essay, Lomberg points out that there is a major timing issue here. Just how long would a beacon like this take before it repeated?
If you ever detect a possible beacon, you have to remain on target long enough for it to repeat—and who knows how long you have to wait? For an ancient and long-lived society, with perspectives far longer than our 10,000-year civilization, that might be a long time. Their notion of patience might be very different from ours. They’re aliens, after all. Of one thing I am sure: Any brief, potentially artificial signal should be closely watched for a repeat. A new approach to SETI could involve unbroken observation of some of the special directions on the sky.
Given that open question, we still need to maximize the possibilities, and I think Lomberg is right in emphasizing a strategy of constant listening. How to do this? Paul Shuch, the canny and deeply dedicated executive director of the SETI League, has long advocated getting away from what he calls ‘soda straw’ SETI, in which we perform a deep study of a target for only a short period of time before moving on. Instead, Shuch backs attempts like Project Argus, the SETI League’s microwave SETI effort aimed at providing continuous, full-sky coverage.
The notion here is to deploy and coordinate about 5000 small radio telescopes around the world, an attempt to provide continuous monitoring of the sky in real time. A station for Project Argus is not a huge dish but an amateur installation fully capable of detecting a Benford beacon’s transient signal if it should occur. In terms of cost, such a site has more in common with amateur radio than with Arecibo-style astronomy. It could be built for no more than a few thousand dollars and, depending on the builder, perhaps for less.
Image: H. Paul Shuch, N6TX, uses the SETI Horn of Plenty antenna for portable operations when away from his Project Argus station FN11lh. The horn, which fits in the back of a minivan, is ideal for classroom demonstrations, exhibiting +20 dBi of gain at 1.4 GHz. Credit: SETI League.
And yes, the famous “Wow Signal,” a one-off detection via Ohio State’s Big Ear radio telescope in 1977, does seem to fit the model of a Benford beacon, in being powerful, brief, and never seen again. How should we look for the next “Wow Signal”? Science fiction author David Brin backs the Project Argus idea in The Search for Extraterrestrial Intelligence (SETI) and Whether to Send “Messages” (METI):
Clearly SETI would benefit from a supplementary system that covers the Earth, searching continuously and broadly for pings that are sent by ETCs narrowly. That system would be ready to detect and pounce upon any new Wow Signals and automatically net-notify larger telescopes to zoom quickly on the source. Not a competitor with classic SETI, this second layer could serve as an ideal alert-generating system, filling a glaring deficit in the current approach.
An Expanded Project Argus
In Greek mythology, Argus was a giant whose epithet “all seeing” (panoptes) spawned depictions of him with multiple eyes. Argus always had a few eyes open, thus becoming the perfect watchman. Can we find a way to maximize the potential of Project Argus?
For while the endeavor is loaded with promise and benefits from the skills and energies of people like Shuch, it has been unable to reach anything like the needed 5000 stations for continuous coverage. This is why a comment by the above-quoted David Brin on a SETI-oriented mailing list recently caught my eye. Brin notes that even as ‘soda straw’ SETI continues, we have the option of energizing the Project Argus idea. We already see wealthy people like Paul Allen and Yuri Milner becoming deeply involved in SETI. What if we could find a similar figure to create a Project Argus kit?
The idea here would be to take the building of a home receiving station for SETI out of the realm of sophisticated electronic technology and into into a turn-key kit that could be purchased for several hundred dollars and simply attached to a basic backyard dish. Like SETI@Home, a Web-based collection system could be used to track the ongoing datastream. A global system for transient detections like this is the kind of network that could find a Benford beacon.
Image: My friend Mike Gingell, KN4BS, shows off his two dishes, 12 and 10 feet in diameter, used for radio astronomy, satellite TV, and of course SETI. I’m sorry to say that Mike passed away last year, but he remained fascinated by SETI prospects until the end. Credit: Mike Gingell / SETI League.
We need to keep an eye on the possibilities that can emerge from private funding and the work of skilled amateur radio astronomers. But we also need to grow the numbers of those who have the means to participate. An updated version of Project Argus could supplement and extend the original, taking what began as a superb idea for engineers working with home equipment into the realm of everyday users with a yen to use digital tools to explore SETI’s possibilities. Make the kit cheap enough and straightforward to operate and the transient detection system we need emerges, an approach to SETI that widens our capabilities even as traditional SETI continues.
The papers are Benford, “Messaging with Cost Optimized Interstellar Beacons,” Astrobiology Vol. 10(5) (June, 2010), pp. 475-490 (preprint) and “Searching for Cost Optimized Interstellar Beacons,” Astrobiology Vol. 10(5) (June, 2010), pp. 491-498 (preprint). Paul Shuch’s Searching for Extraterrestrial Intelligence: SETI Past, Present, and Future (Springer, 2011) is an essential resource on SETI issues.
Catching Up with Dawn at Ceres
The Dawn spacecraft has reached its final orbital altitude, closing to within 385 kilometers of the asteroid (and yes, I really should start calling Ceres a ‘dwarf planet’ consistently — working on it). We have no observations from this distance yet, but that process begins within days, and should give us images with a resolution of 35 meters per pixel, along with a wealth of data from the craft’s scientific package.
Like New Horizons, Dawn makes history every time it returns observations of places we haven’t seen before, or surface features we’re seeing at higher resolution as the orbit lowers. Unlike New Horizons, Dawn is an orbiter, which makes me long for the idea of a Pluto orbiter, even though New Horizons has amply demonstrated how useful and powerful a flyby mission can be. An orbiter lets you complete the mapping process so essential to making a new world tangible, while there are parts of Pluto that our flyby couldn’t make out at highest resolution.
I found the temperatures Dawn recorded at Ceres a bit startling. The 180 K (-93 °C) on the low side seems about right, but I hadn’t expected equatorial temperatures to reach as high as 240 K (-33 °C), which is a temperature I can recall experiencing several times in Iowa during an unusually tough winter. This JPL news release notes that temperatures at and near Ceres’ equator are too high to support surface ice for long periods, as explained in new work published in Nature to which we now turn.
Maria Cristina De Sanctis (National Institute of Astrophysics, Rome) and colleagues tell us in one of the two new papers that Dawn has turned up evidence for clays that are rich in ammonia on Ceres, using data gathered by the spacecraft’s visible and infrared mapping spectrometer. I hearken back to those temperatures because Ceres is too warm to support surface ammonia ice, but ammoniated compounds (ammonia molecules combining with other minerals) could be stable. Finding these tells us that Ceres may not have formed in the main asteroid belt.
“The presence of ammonia-bearing species suggests that Ceres is composed of material accreted in an environment where ammonia and nitrogen were abundant,” says De Sanctis. “Consequently, we think that this material originated in the outer cold solar system.”
The other possibility: The dwarf planet formed about where it is today but drew in materials from the outer system that had formed near the orbit of Neptune. Another interesting finding: Although carbonaceous chondrites (meteorites rich in carbon) are thought to be similar to Ceres in composition, the data do not match at all wavelengths. Ceres shows absorption bands in its reflected light that match up with the ammoniated minerals described above. Moreover, Ceres shows water content as high as 30 percent, while carbonaceous chondrites normally weigh in at 15 to 20 percent bulk water content, possibly indicating accretion from volatile-rich material.
Image: Dwarf planet Ceres is shown in these false-color renderings, which highlight differences in surface materials. Images from NASA’s Dawn spacecraft were used to create a movie of Ceres rotating, followed by a flyover view of Occator Crater, home of Ceres’ brightest area. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Resolving the Bright Spots
While we began seeing a small number of unusual bright spots on Ceres as Dawn approached, it turns out that close study reveals more than 130 areas of unusual brightness, most of them associated with impact craters. The second paper in Nature is the work of Andreas Nathues (Max Planck Institute for Solar System Research, Göttingen) and colleagues. Here we learn that the bright material is consistent with hexahydrite, which is a type of magnesium sulfate. What the paper argues is that the bright spots are areas rich in salt that were left behind when water ice sublimated long ago, having been brought to the surface by an impact.
Image: This representation of Ceres’ Occator Crater in false colors shows differences in the surface composition. Red corresponds to a wavelength range around 0.97 micrometers (near infrared), green to a wavelength range around 0.75 micrometers (red, visible light) and blue to a wavelength range of around 0.44 micrometers (blue, visible light). Occator measures about 90 kilometers wide. Scientists use false color to examine differences in surface materials. The color blue on Ceres is generally associated with bright material, found in more than 130 locations, and seems to be consistent with salts, such as sulfates. It is likely that silicate materials are also present. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
That, of course, backs up the idea that we’re dealing with a subsurface layer of salty water ice, an idea supported by the global nature of the bright spots. The Occator crater contains the brightest material found on Ceres, and it also appears to be one of the youngest surface features, with an age of about 78 million years. Remarkably, what appears to be a diffuse haze can be seen filling the floor of the crater at certain times of day.
The haze appears to be absent at dawn and dusk, while it can be seen at local noon, making Ceres, in the view of the study authors, something like a comet, where water vapor when warmed can lift particles of dust and residual ice off the surface. The Herschel space observatory reported water vapor at Ceres in 2014, a finding consistent with these observations. Remember, however, that we’re still waiting on the unambiguous detection of water ice on Ceres, so the story of the Occator haze will require more data and further analysis.
The papers are Nathues et al., “Sublimation in bright spots on (1) Ceres,” Nature 528 (10 December 2015), 237-240 (abstract) and De Sanctis et al., “Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres,” Nature 528 (10 December 2015), 241-244 (abstract).
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